1. Field of the Invention
Embodiments of the present general inventive concept relate to a fixing device having improved safety against overheating and an image forming apparatus having the same.
2. Description of the Related Art
Generally, an image forming apparatus is provided with a fixing device which heats a paper having a powdery toner image transferred thereon so that the toner image is temporarily fused and thereby fixed onto the paper. The fixing device typically comprises a fixing roller that fixes the toner image to the paper, and a pressing roller that supports and presses the paper against the fixing roller. The fixing roller typically includes a heating lamp mounted through the inner center thereof to heat the fixing roller with radiant heat from the heating lamp.
A thermistor, which contacts an outer circumference of the fixing roller and measures a surface temperature of the fixing roller, and a thermostat, which interrupts power being supplied to the heating lamp when the surface temperature of the fixing roller exceeds a predetermined threshold, are typically installed at one side of the fixing roller. The thermistor transmits the measurement result to a controller (such as a microcomputer) of the image forming apparatus, and the controller switches a switching element based on the measured temperature, thereby controlling power supplied to the heating lamp so that the surface temperature of the fixing roller can be maintained within a certain range. When the surface temperature is higher than the predetermined threshold, the thermostat interrupts the power supply to the heating lamp by opening an internal terminal.
However, conventionally, if an A/D port of the controller begins to perform erratically due to certain external factors such as static electricity, a voltage may be continuously applied by the A/D port. If the switching element is kept on continuously due to a short circuit, on and off states of the heating lamp may not be normally controlled. In such cases, the fixing roller is overheated because the AC power is continuously applied to the heating lamp until the surface temperature of the fixing roller is increased to over the predetermined threshold and the internal terminal of the thermostat is opened. Accordingly, peripheral parts including the fixing roller may be melted or damaged by the high temperature.